This interesting conversation between the Founder of ISCKON A.C. Bhaktivedanta Swami Prabhupada and an Interviewer in London explains a very simple essential logic in order to select Guru.
Just like before buy a new gadget or any expensive item we first do some research about the product then go to shop talk to experts in order to do purchase.Similarly before selecting Spiritual Guru we need to know basic knowledge of Spirituality